Over-driven access method and device for ferroelectric memory

ABSTRACT

An over-driven access method and device for ferroelectric memory. When accessing the data stored in a ferroelectric memory, the invention further provides an over-driven current to slightly reduce/raise the voltages in bit lines BL and BL′ to further enlarge the voltage difference therebetween after having raised the plate-line/bit-line voltage using the plate-line/bit-line driven method.

This application claims the benefit of Taiwan application Serial No.92108988, filed Apr. 17, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to a memory access method and device,and more particularly to an over-driven access method and device forferroelectric memory.

2. Description of the Related Art

Ferroelectric memory uses ferroelectric capacitor as the storage medium.Ferroelectric capacitor can be polarized to different polarized states,and thus can represent the stored data by means of polarized states.Please refer to FIG. 1, a transitional diagram illustrating thetransition of the polarized state of ferroelectric capacitor. Thetransition of the polarized state of ferroelectric capacitor ishysteresis which means that a greater-than-zero voltage must be appliedin order to convert the polarization of a ferroelectric capacitor from anegative state into a positive state and that a smaller-than-zerovoltage must be applied in order to convert the polarization of aferroelectric capacitor from a positive state into a negative state.Seeing that the polarized state of ferroelectric capacitor still can besustained even in the absence of power supply, the ferroelectriccapacitor does have the potential to replace the flash memory which isin current use now. Moreover, the polarized state of ferroelectriccapacitor can be changed when the voltage applied onto the ferroelectriccapacitor changes, thus has the potential to replace the dynamic randomaccess memory (DRAM) as well.

Apart from having the characteristic of hysteresis, the ferroelectriccapacitor has another characteristic, i.e., its capacitance ischangeable. The formula for capacitance C is: C=ΔQ/ΔV, wherein Q is thequantity of electric charges; Q is the voltage drop of capacitor. Thepolarization of ferroelectric capacitor P is proportional to thequantity of electric charges Q, therefore capacitance C is proportionalto the slope of the transition curve for polarization in FIG. 1. Thelarger the slope is, the greater the capacitance will be. It can beinferred from FIG. 1 that the capacitance during polarity transition islarger than that in a stabilized state.

Please refer to FIG. 2A, a schematic diagram illustrating a memory unitof a ferroelectric memory. This memory unit which is in the form of 1T1Cconsists of one transistor, T, and one ferroelectric capacitor, Cf, withone end of ferroelectric capacitor Cf being coupled to plate line PL.When word line WL is enabled, the voltage drop in ferroelectriccapacitor Cf is exactly the voltage difference between plate line PL andbit line BL. Please refer to FIG. 2B, a schematic diagram illustratinganother memory unit of a ferroelectric memory. This memory unit which isin the form of 2T2C consists of two transistor, T and T′, and twoferroelectric capacitors, Cf and Cf′. Every memory unit furthercomprises a sense amplifier SA which is used to amplify the voltagedifference between bit line BL and bit line BL′ to facilitate the accessto the data stored at the ferroelectric memory. Sense amplifier SA asillustrated in FIG. 2B is a latch sense amplifier further comprising twophase inverters which raises the voltage in the high-voltage bit line tobe even higher and reduces the voltage in the low-voltage bit line to beeven lower. Sense amplifier SA is initiated when sense amplifierenabling signal SAE is received.

General speaking, there are two methods for the access of ferroelectricmemory: the plate-line driven method and the bit-line driven method.Please refer to FIG. 3, a timing chart when plate-line driven method isused to access the ferroelectric memory using the ferroelectric memoryin FIG. 2 as an example. At the initial stage, P and P′, the polarizedstate of ferroelectric capacitor Cf and that of ferroelectric capacitorCf′, are of positive polarity and negative polarity respectively.Firstly, enable word line WL during period T1 such that transistors Tand T′ can be conducted. Next, enable plate line PL during period T2 andraise the voltage in plate line PL to high level. Since the voltage dropof ferroelectric capacitors Cf and Cf′ has changed to be positive, theirpolarized states P and P′ as shown in the diagram become positive aswell. With the change in polarity from negative to positive, thecapacitance of ferroelectric capacitor Cf′ becomes larger and storesmore electric charges causing bit line BL′ to have a higher voltageaccordingly. Following that, enable sensing g amplifier SA to enlargethe voltage difference between bit line BL and bit line BL′ tofacilitate the identification of the data stored at the memory unit.Meanwhile, the voltage in bit line BL′ whose voltage is higher will beraised to high level; the voltage in bit line BL whose voltage is lowerwill be reduced to low level. The voltages in bit line BL′ and thevoltage in plate line PL are both at high levels, so the voltage drop inferroelectric capacitor Cf′ nears zero. The transition for the polarizedstate P which still remains positive is shown in the diagram. Theabovementioned period T1 and T2 are driving stage during which time bitline voltages are differentiated; period T3 is sensing stage.

During period T3, P′, the polarity of ferroelectric capacitor Cf′ whichwas originally negative prior to accessing stage, becomes positive. Inorder to restore the original polarized state for ferroelectriccapacitor Cf′, some recovery procedures need to be taken. First, disableplate line PL during period T4, so that P′, the polarized state offerroelectric capacitor Cf′, will become negative for the voltage dropin ferroelectric capacitor Cf′ has already turned negative. Next,disable sense amplifier SA during period T5 such that the voltage in bitline BL will drop immediately causing the voltage drop in ferroelectricCf′ to be reduced to zero. The original polarized state will thus berestored.

The plate-line driven method enables plate line PL and word line WLfirst causing bit lines BL and BL′ to generate different voltagesaccording to the polarized states of ferroelectric capacitors Cf andCf′. The bit-line driven method enables bit lines BL and BL′ beforeenabling word line WL, so that bit lines BL and BL′ can generatedifferent voltages according to the polarized states of ferroelectriccapacitors Cf and Cf′. This has been understood by those who areconventional with this technology and will not be repeated here.

In order to further improve access quality, an over-driven access methodis proposed in “A 76 mm2 8 Mb Chain Ferroelectric Memory” (ISSCC, 2001).Please refer to FIG. 4, a schematic diagram for ferroelectric memory 400according to a conventional overdriven access method. Ferroelectricmemory 400 differs from the abovementioned ferroelectric memories inthat bit lines BL and BL′ are respectively connected to transistors Qand Q′ and to capacitors Cov and Cov′. Please refer to FIG. 5, a timingchart according to a conventional over-driven method. Suppose thepolarized states of cells CL and CL′ are negative and positiverespectively at the initial stage. The over-driven access method is usedto assist the abovementioned plate-line driven method or bit-line drivenmethod. In the present example, over-driven access method is used toassist plate-line driven method. First, raise the voltage in plate linePL at time t1. Since the polarity of cell CL has already changed fromnegative to positive, cell CL will have a larger capacitance and havemore electric charges to be coupled to bit line BL. Consequently, thevoltage in bit line BL will be higher than that in BL′ with a voltagedifference of Δv1. Next, reduce over-driven voltage ODV at time t3, sothat the voltages in bit lines BL and BL′ will drop accordingly. Owingto a larger capacitance in cell CL, the voltage in bit line BL will dropslower than that in bit line BL′. Due to the introduction of over-drivenvoltage OVD, Δv2, the voltage difference between bit line BL and bitline BL′, is enlarged and is greater than Δv1. Last, initiate senseamplifier SA at time t5 to raise the voltage in bit line BL to highlevel and reduce the voltage in bit line BL′ to low level. The contentsstored at the present memory unit will be able to be accessed. However,more capacitors and transistors are required in this method resulting inan increase in chip size and manufacturing cost as well.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an over-drivenaccess method and device which does not require any increase in chipsize to improve access efficiency. When accessing the data stored in aferroelectric memory, the invention further provides an over-drivencurrent to further enlarge the voltage difference between bit line BLand bit line BL′ after having raised the plate line voltage whenplate-line driven method is applied or having raised the bit linevoltage if the bit-line driven method is applied.

It is another object of the invention to provide a ferroelectric memorycharacterized by using a first current source and a second currentsource as over-driven current supplies.

It is another object of the invention to provide a plate-lineover-driven access method. First, raise the plate line voltage to have avoltage difference generated between a positive bit line and a negativebit line. Next, enlarge this voltage difference by providing a firstleakage current and a second leakage current to the positive bit lineand the negative bit line respectively such that the voltages in thepositive and the negative bit lines can be reduced. After that, enable asense amplifier of the ferroelectric memory to further enlarge thevoltage difference. Last, sense the voltage difference between thepositive and the negative bit lines and output the contents stored atthe ferroelectric memory accordingly.

It is another object of the invention to provide a bit-line over-drivenaccess method. First, pre-charge the positive and the negative bit linessuch that their voltages can be raised to high levels. Next, enable theword line such that a voltage difference between the positive bit lineand the negative bit line occurs. After that, enlarge the voltagedifference by providing a first leakage current and a second leakagecurrent to the positive bit line and the negative bit line respectively.After that, enable a sense amplifier of the ferroelectric memory tofurther enlarge the voltage difference. Last, sense the voltagedifference between the positive bit line and the negative bit line andoutput the content stored at the ferroelectric memory accordingly.

It is another object of the invention to provide a ferroelectric memorywhich uses the plate-line driven access method or the bit-lineover-driven access method. The sense amplifier, which is controlled byswitch P and switch N, is used as a current source when only one of thetwo switches is enabled and will provide normal functions of sensing andamplification when the two switches P and N are both enabled.

Other objects, features, and advantages of the invention will becomeapparent from the following detailed description of the preferred butnon-limiting embodiments. The following description is made withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transitional diagram illustrating the transition ofpolarized state for a ferroelectric capacitor;

FIG. 2A is a schematic diagram illustrating a memory unit of aferroelectric memory;

FIG. 2B is a schematic diagram illustrating another memory unit of aferroelectric memory;

FIG. 3 is a timing chart when plate-line method is applied in aferroelectric memory;

FIG. 4 is a conventional schematic diagram when over-driven accessmethod is applied in a ferroelectric memory;

FIG. 5 is a conventional timing chart when over-driven method is appliedin a ferroelectric memory;

FIG. 6A is a schematic diagram for a ferroelectric memory usingplate-line over-driven access method according to embodiment one of theinvention;

FIG. 6B is a schematic diagram for a ferroelectric memory illustrated inFIG. 6A;

FIG. 7 is a timing chart when plate-line over-driven method is appliedin a ferroelectric memory;

FIG. 8 is a schematic diagram for a ferroelectric memory usingover-driven access method according to embodiment one of the invention;

FIG. 9 is a timing chart when plate-line over-driven method is appliedin a ferroelectric memory illustrated in FIG. 8;

FIG. 10A is a schematic diagram for a ferroelectric memory usingbit-line over-driven access method according to embodiment two of theinvention;

FIG. 10B is a schematic diagram for a ferroelectric memory illustratedin FIG. 10A;

FIG. 11 is a timing chart when bit-line over-driven access method isapplied in a ferroelectric memory illustrated in FIG. 10B; and

FIG. 12 is a timing chart when bit-line over-driven access method isapplied in a ferroelectric memory illustrated in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

The spirit of the invention is that when accessing the data stored in aferroelectric memory, the invention further provides an over-drivencurrent to further enlarge the voltage difference between bit line BLand bit line BL′ after having raised the plate line voltage whenplate-line driven method is applied or having raised the bit linevoltage if the bit-line driven method is applied. The plate-lineover-driven method which assists plate-line driven method and thebit-line over-driven method which assists bit-line driven method aredisclosed below respectively.

Embodiment One

Please refer to FIG. 6A, a schematic diagram for ferroelectric memory600 using plate-line over-driven access method according to embodimentone of the invention. What ferroelectric memory 600 differs from aconventional memory unit is that bit lines BL and BL′ are provided withcurrent sources I1 and I1′ respectively which can be implemented byP-type transistors as shown in FIG. 6B, a schematic diagram forferroelectric memory 650 in FIG. 6A. In FIG. 6B, current sources I1 andI1′ are implemented by N-type transistors Q1 and Q1′; the conduction oftransistors Q6 and Q6′ is determined according to the level ofover-driven voltage ODV.

Please refer to FIG. 7, a timing chart when plate-line over-drivenmethod is applied in ferroelectric memory 650. Suppose thatferroelectric capacitors Cf and Cf′ are initially of negative polarityand of positive polarity respectively. First, raise the voltage in plateline PL at time t1. At which time, ferroelectric capacitor Cf will havea larger capacitance because its polarized state has been converted fromnegative polarity into positive polarity allowing more electric chargesto be coupled to bit line BL. Consequently, bit line BL will have ahigher voltage than bit line BL′ with a voltage difference of Δv1. Next,reduce over-driven voltage ODV at time t3, so that the voltages in bitlines BL and BL′ will drop accordingly. Since ferroelectric capacitor Cfhas a larger capacitance than ferroelectric capacitor Cf′, the voltagein bit line BL will drop slower than the voltage in bit line BL′. Owingto the introduction of overdriven voltage OVD, Δv2, the voltagedifference between bit line BL and bit line BL′, is enlarged and isgreater than Δv1. Last, initiate sense amplifier SA at t5 to raise thevoltage in bit line BL to high level and reduce the voltage in bit lineBL′ to low level. The contents stored at present memory unit will beable to be accessed.

Like the conventional ferroelectric memory shown in FIG. 2, the abovedisclosed ferroelectric memories which use over-driven access method asshown in FIG. 6A and FIG. 6B still require additional elements. However,the ferroelectric memories in FIG. 6A and FIG. 6B require smallerincrease in chip size than the ferroelectric memory using over-drivenaccess method in FIG. 4 does. Therefore, the ferroelectric memoryaccording to the invention is able to reduce the required size for thechip. Another method which does not require any additional element isdisclosed below.

Please refer to FIG. 8, a schematic diagram for a ferroelectric memoryusing over-driven access method according to embodiment one of theinvention. Since ferroelectric memory 800 uses the existing senseamplifier SA as the source of currents, not any additional element isrequired. Sense amplifier SA is a latch sense amplifier comprisingN-type transistors Qn, Qn′ and Qsn, and P-type transistors Qp, Qp′ andQsp. Of which, transistor Qsn is controlled by controlling signal SANwhile transistor Qsp is controlled by controlling signal SAP;transistors Qn and Qp form a set of phase inverter while transistors Qn′and Qp′ form another set of phase inverter. Please refer to FIG. 9, atiming chart when plate-line over-driven method is applied in aferroelectric memory illustrated in FIG. 8. Suppose that P and P′, thepolarized states of ferroelectric capacitors Cf and Cf′, are initiallyof positive polarity and of negative polarity respectively. Enable wordline WL to have transistors T and T′ be conducted during period T1; andenable plate line PL to have the voltage in plate line PL be raised tohigh level during period T2. Since the voltage drop in ferroelectriccapacitors Cf and Cf′ has become positive, their polarized states P andP′ as shown in the diagram are positive as well. Ferroelectric capacitorCf′ will have a larger capacitance and carry a larger number of electriccharges causing the voltage in bit line BL′ to raise when the polarizedstate of ferroelectric capacitor Cf′ changes from negative intopositive. Over-driven current is provided during period T3-1. Duringwhich time, enable controlling signal SAN to have transistor Qsnconducted. Since bit lines BL and BL′ have greater-than-zero voltages,the electric charges in BL and BL′ can flow out through transistors Qn,Qn′ and Qsp causing the voltages in bit lines BL and BL′ to drop down.The voltage in bit line BL almost does not drop at all because Cf, theelectric capacitor which is connected to bit line BL, has a largercapacitance than Cf′ has, but the voltage in bit line BL′ drops moresignificantly. As a consequence, the voltage difference between bit lineBL and bit line BL′ is enlarged and hence the object of the invention isachieved. Next, enable controlling signal SAP during period T3-2. Duringwhich time, sense amplifier SA is enabled, so the voltage in bit lineBL′ is raised to high level while that of bit line BL is reduced to lowlevel. Now the contents stored at memory unit can be identifiedaccording to the voltage difference between bit line BL and bit lineBL′.

Embodiment Two

Please refer to FIG. 10A, a schematic diagram for ferroelectric memory1000, a ferroelectric memory using bit-line over-driven access methodaccording to embodiment two of the invention. Ferroelectric memory 1000differs from a conventional memory unit in that bit lines BL and BL′ areprovided with current source I2 and current source I2′ respectively,wherein the source of currents can be implemented by P-type transistorsas shown in FIG. 10B. FIG. 10B is a schematic diagram for ferroelectricmemory 1050 illustrated in FIG. 10A. Of which, the function of currentsources I2 and I2′ is achieved via the use of P-type transistors Q2 andQ2′; the conduction of transistors Q2 and Q2′ is determined according tothe level of over-driven voltage ODV.

Please refer to FIG. 11, a timing chart when bit-line over-driven accessmethod is applied in ferroelectric memory 1050 illustrated in FIG. 10B.Suppose that ferroelectric capacitors Cf and Cf′ are initially ofnegative polarity and of positive polarity respectively. First, chargebit lines BL and BL′ until their voltages are raised to high levels attime t1. Next, enable word line WL at time t2. Since transistor T and T′are conducted now, the electric charges in bit lines will flow toferroelectric capacitors Cf and Cf′. The voltage in bit line BL dropsfaster than the voltage in bit line BL′ does because ferroelectriccapacitor Cf has a capacitance larger than that of Cf′ resulting in avoltage difference of Δv1 between bit line BL and bit line BL′. Afterthat, raise over-driven voltage ODV to high level to have transistors Q2and Q2′ conducted at time t3. The voltages in bit lines BL and BL′ willincrease due to the inflow of electric charges originating from thesource of currents. The voltage increase in bit line BL is insignificantbecause ferroelectric capacitor Cf has a capacitance larger than that offerroelectric capacitor Cf′, but the voltage increase in bit line BL′ ismore significant than that in bit line BL because the capacitance offerroelectric capacitor Cf′ is smaller than that of ferroelectriccapacitor Cf resulting in a larger Δv2, the voltage difference betweenbit line BL and bit line BL′. Hence the object of the invention isachieved. At time t4, initiate sense amplifier SA to have the voltage inbit line BL be reduced to low level and to have the voltage in bit lineBL′ be raised to high level. Now the data stored at the memory unit canbe accessed accordingly.

Like the conventional ferroelectric memory illustrated in FIG. 2B, theferroelectric memory using bit line over-driven access method asillustrated in FIG. 10A and 10B also requires additional elements.However, the required increase in chip size is smaller than theferroelectric memory illustrated in FIG. 4 requires. Therefore theferroelectric memory according to the invention is able to reduce therequired chip size.

Moreover, ferroelectric memory 800 illustrated in FIG. 8 can also usebit line over-driven method to access the stored contents. Please referto FIG. 12, a timing chart when bit-line over-driven access method isapplied in ferroelectric memory 800 illustrated in FIG. 8. Suppose thatferroelectric capacitors Cf and Cf′ are initially of positive polarityand of negative polarity respectively. First, charge bit lines BL andBL′ during period T1. Next, during period T2 enable word line WL to havetransistors T and T′ be conducted so that the electric charges in thebit lines flow into ferroelectric capacitors Cf and Cf′. The voltage inbit line BL drops faster than that in bit line BL′ because ferroelectriccapacitor Cf has a larger capacitance than ferroelectric capacitor Cf′has resulting in a voltage difference of Δv1 between bit line BL and bitline BL′. After that, during period T3-1, reduce controlling signal SAPso that transistor Qsp can be conducted allowing electric charges toflow into bit lines BL and BL′ from the source of currents. The voltageincrease in bit line BL is insignificant because ferroelectric capacitorCf has a large capacitance, but the voltage increase in bit line BL′ ismore significant because ferroelectric capacitor Cf′ has a smallcapacitance resulting in a larger Δv2, the voltage difference betweenbit line BL and bit line BL′. Hence the object of the invention isachieved. After that, raise controlling signals SAN during period T3-2.Sense amplifier SA is now fully enabled allowing the voltage in bit lineBL to be reduced to low level and the voltage in bit line BL′ to beraised to high level. The contents stored at the memory unit can now beaccessed. During periods T4 and T5 the polarized states of ferroelectriccapacitors Cf and Cf′ are restored and will not be repeated here.

The ferroelectric memory using over-driven access method and device asdisclosed in above embodiments can increase accessing efficiency underthe circumstance of a reduced chip size.

While the invention has been described by way of example and in terms ofa preferred embodiment, it is to be understood that the invention is notlimited thereto. On the contrary, it is intended to cover variousmodifications and similar arrangements and procedures, and the scope ofthe appended claims therefore should be accorded the broadestinterpretation so as to encompass all such modifications and similararrangements and procedures.

1-10. (canceled)
 11. A ferroelectric memory comprising: a memory unitcomprising: a positive bit line and a negative bit line which areparallel to each other and are coupled to the sense amplifier; a wordline which is virtually perpendicular to the positive and the negativebit lines; a positive memory cell which is coupled to the word line andthe positive bit line and will be connected to the positive bit linewhen the word line is enabled; a negative memory cell which is coupledto the word line and the negative bit line and will be connected to thenegative bit line when the word line is enabled; and a plate line whichis coupled to the positive and the negative memory units; and a senseamplifier, comprising: a first phase inverter whose outlet and inputends are coupled to the positive and the negative bit linesrespectively, outputting the voltage in the negative bit line afterhaving inverted the phase of the voltage in the negative bit line,comprising a first P-type transistor and a first N-type transistor,wherein the gate electrodes of the first P-type transistor and the firstN-type transistor are coupled to the negative bit line while the drainof the first P-type transistor and the source electrode of the firstN-type transistor are coupled to the outlet end of the first phaseinverter; a second phase inverter whose input and outlet ends arecoupled to the positive and the negative bit line respectively,outputting the voltage in the positive bit line after having invertedthe phase of the voltage in the positive bit line, comprising a secondP-type transistor and a second N-type transistor, wherein the gateelectrodes of the second P-type transistor and the second N-typetransistor are coupled to the positive bit line while the drain of thesecond P-type transistor and the source electrode of the second N-typetransistor are coupled to the outlet end of the second phase inverter; aP-switch with one end coupled to the source electrodes of the first andthe second P-type transistors and the other end coupled to a powersource; and an N-switch with one end coupled to the drains of the firstand the second N-type transistors and the other end grounded.
 12. Theferroelectric memory according to claim 11, wherein the ferroelectricmemory is applied in a plate-line driven access method which comprisesthe steps of: enabling the word line; enabling the plate line togenerate a voltage difference between the positive bit line and thenegative bit line; enabling the N-switch to enlarge the voltagedifference; enabling the P-switch to have the sense amplifier be enabledto further enlarge the voltage difference by raising the higher voltageamong the voltage in the positive bit line and the voltage in thenegative bit line to high level but reducing the lower voltage to lowlevel; and sensing the voltage difference between the positive bit lineand the negative bit line and outputting the content stored at theferroelectric memory accordingly.
 13. The ferroelectric memory accordingto claim 11, wherein the ferroelectric memory is applied in a bit-linedriven access method which comprises the steps of: pre-charging thepositive and the negative bit lines; enabling the word line to generatea voltage difference between the positive bit line and the negative bitline; enabling the P-switch to enlarge the voltage difference; enablingthe N-switch to have the sense amplifier be enabled to further enlargethe voltage difference; and sensing the voltage difference between thepositive bit line and the negative bit line and outputting the contentstored at the ferroelectric memory accordingly.
 14. The ferroelectricmemory according to claim 11, wherein switch P is a P-type transistor.15. The ferroelectric memory according to claim 1 1, wherein switch N isan N-type transistor.